Endoscopic instruments are widely known in which illumination of the field is provided by a fiberoptic cable or bundle that transmits light through the barrel of the endoscope to its distal end and in which visual images are then transmitted back to the physician through a series of lenses extending through the endoscope barrel from a field lens at the distal end to an eyepiece lens assembly at the endoscope's proximal end. The field lens commonly faces in a forward or longitudinal direction, although a forward-oblique orientation is often provided. Where lateral viewing is desired, the practioner must usually remove the endoscope from the patient and replace it with another endoscope having side-viewing capability. It is believed apparent that endoscopic procedures would be rendered more effective and efficient if a single endoscope could be arranged so that the user could switch at will between two views (forward and lateral) of the inspection site without removing the endoscope from the patient.
One aspect of this invention lies in the recognition of such a need and the further recognition that mechanical schemes for achieving such an objective would not be practical because the mechanisms would be too complicated, would occupy too much space, and would be nearly impossible to fabricate economically. Such considerations render unrealistic any construction in which, for example, an endoscope might be provided with a mechnically-steerable prism and field lens assembly, or with two field lenses in combination with a prism that is somehow moved mechanically as to direct one or the other of the two fields into the objective lens.
In brief, this invention involves the discovery that a relatively uncomplicated and highly-effective dual view endoscope may be achieved without any mechanical switching mechanism, or any separately movable parts, at the endoscope's distal end. Such results are accomplished by providing the endoscope with two fixed field lens assemblies at the endoscope's distal end, one assembly facing generally forwardly and the other laterally. Light received by the field lenses is directed through a beam combiner prism and then through an objective lens and a series of relay lenses back to the eyepiece lens assembly at the endoscope's proximal end. A control system is provided for selectively blocking the transmission of light received by either of the field lens assemblies while allowing the transmission of light received by the other of those assemblies. In one embodiment, the adjustable light transmission control means takes the form of polarizing filters for polarizing the light passing from the field lenses so that the orthological linear polarization of light from one of the field lens assemblies is at right angles to the polarization of light from the other assembly, and then providing the endoscope with an adjustable polarization-sensitive transmission filter interposed between the field lenses and the eyepiece for selectively blocking the transmission of polarized light from either one of the field lenses while transmitting polarized light from the other of such field lenses. In another embodiment, the light transmission control means takes the form of a pair of liquid crystal filters interposed between the respective field lenses and the beam combiner prism, each of the liquid crystal filters being capable of being electrically activated to permit light from only a selected one of the two field lenses to enter the beam combiner prism at any one time.
Where polarization is used for selectively controlling the transmission of an image from one field lens or the other, the light-polarizing means may comprise dichroic polarizers, or other suitable polarizing elements, at the exit faces of the field lenses. In a preferred form, selective polarization is produced by a multilayer dielectric reflector in the beam combiner prism. The polarization-sensitive transmission filter may take the form of a dichroic polarization filter, or a thin-film multilayer polarization filter, or a MacNeille prism. Ninety degree rotation of the filter or prism within the proximal end portion of the endoscope may be achieved by manually rotating an external collar that is mechnically or magnetically coupled to the polarization-sensitive transmission filter. Alternatively, that filter may be fixed within the endoscope and a twisted nematic liquid crystal cell having glass plate electrodes may be used for electrically controlling the transmission of an image from one field lens assembly or the other.
Other advantages, features, and objects of the invention will become apparent from the specification and drawings.